1. Field of the Invention
The invention relates to the field of electrophotography and more particularly to a method and apparatus for applying an electrostatic charge to a moving electrophotographic film.
2. Description of the Prior Art
In electrophotography, it is common to apply a uniform electrostatic charge to the surface of a photoconductive layer. The charge in selected areas is then dissipated in a pattern by exposing the surface to a light image to form an electrostatic latent image. The latent image is then rendered visible by applying thereto finely divided electrostatically charged developer particles which adhere to the surface by electrostatic attraction. Permanent visible images can be obtained, for example, by using thermoplastic developer particles which are heat fused to the photoconductive layer.
Charging is conventionally accomplished by exposing the surface of the photoconductive layer to a corona source, the polarity of which is chosen to produce the desired results upon the particular photoconductive layer being charged. The corona source is commonly provided by one or more fine wires positioned close to the photoconductive layer. When a high voltage potential is applied to the wire or wires, a corona is generated or discharged and ions are attracted to and deposited on the surface of the photoconductive layer. Superior image reproductions are obtainable only when very uniform electrostatic charges are established on the photoconductive layer before imaging.
High voltages for generating corona are particularly desirable for producing charge uniformity, but can subject the photoconductive layer of the film to excessive charge build-up (charge potential), which can damage the photoconductive layer by current leakage into the conductive layer beneath. A number of techniques have been employed to limit the charge potential on the photoconductive layer. For example, complex electrical circuitry has been used to limit corona production (an example being disclosed in U.S. Pat. No. 3,335,275 to King).
A number of problems which are easily solved when charging a moving film when the apparatus is designed to move the film at only one speed are more difficult to solve when the film speed is varied. For example, if the film is moved too slowly, it tends to overcharge, resulting in voltage leaking through the photoconductive layer and damaging the film. On the other hand, if the film is moved too rapidly past the charging apparatus, insufficient charging results. Varying charge rate could be compensated for by varying the corona voltage, but complex electronic circuitry would be necessary. Additionally, the rate and degree of charging is affected by other factors, such as humidity, which changes as the apparatus warms up.
Another technique employed to limit the charge potential on a film is the use of a wire grid or screen placed between the corona discharge wire and the photoconductive surface. This apparatus is commonly referred to as a "scorotron" and is described in U.S. Pat. No. 2,777,957. The grid is maintained at a predetermined potential and serves to terminate further charging of the photoconductive surface when the charge on all portions of the photoconductive surface corresponds to the grid potential. The grid can be grounded or biased by means of an external voltage source, or it can be self-biased from the corona current by connecting the grid to ground through current flow restricting devices (an example of the latter being illustrated in U.S. Pat. No. 3,729,649).
Using a "scorotron", a high voltage can be applied to the corona discharge wire without fear of overcharging the photoconductive surface. One disadvantage of the "scorotron" grid is the "shadowing" effect which is particularly noticeable when charging a moving recording element and a portion of the grid extends in the direction parallel to the direction of relative movement. Another disadvantage to the "scorotron" is its complexity and cost of manufacture, due to the necessity of providing delicate and uniformly sized members, particularly when using it in small charging systems. It is also difficult to repair and clean and is inefficient in operation because it drains a large amount of corona current away from the film. The "scorotron" is useful for controlling the charge potential on a moving film but it has disadvantages. In addition to those already listed, it encourages arcing between the grid and the portion of the film just moving into the charging or corona zone, since the grid is very close to the film and has a relatively high potential difference between it and the portion of the film entering the corona zone.